In the beginning of the Maharal's Sefer Ner Mitzvah on Chanukah, he starts with a discussion about the four Malchiyos (Kingdoms)/ the four Golios(Exiles). These four Golios were part of the plan of the world since creation, and take over when Klal Yisroel do not shine as brightly as they should. Four represents disunity, as seen in many places in the Torah. These four exiles cause a setback in the perfection of the world, which will be achieved when Klal Yisroel is unified in purpose and can successfully and completely bring Hashem into the creation.